1. Field of the Invention.
This invention relates to the field of electromagnets. More specifically, the invention comprises a tilted Bitter-disk type magnet capable of producing a uniform field which is transverse to the center axis of the coil.
2. Description of the Related Art.
Bitter-disk type electromagnets have been in use for many decades. While it is true that those skilled in the art are familiar with their design and construction, a brief explanation of the prior art will be helpful in understanding the proposed invention.
FIG. 1 shows a prior art Bitter-disk magnet. End plate 12 is the anchoring point for a number of radially-spaced tie rods 16. In practice tie rods 16 have uniform length. Some of these are shown cut away in order to aid visualization of other components. A Bitter-disk magnet is typically constructed by stacking the components. Starting with end plate 12, tie rods 16 are added. A series of conducting disks 18 are then slipped onto tie rods 16. The reader will observe that each conducting disk 18 has a series of holes designed to accommodate tie rods 16. Conducting disks 18 are made of thin conductive material, such as copper or aluminum.
Turning briefly to FIG. 2, the reader may observe conducting disk 18 in more detail. Tie rod holes 24 are uniformly spaced around its perimeter. Cooling holes 26 are also spaced about conducting disk 18. These holes are sometimes made as elongated slots in more complex patterns to optimize both cooling and mechanical strength. As they are not important features of the present invention, however, they have been illustrated simply. In order to avoid visual clutter, the cooling holes have not been illustrated at all in FIG. 1.
FIG. 2 shows cut 22 in conducting disk 18. This is a radial cut extending completely through one side of the disk. The reader will observe that the two sides of the disk have been displaced vertically, with the result that conducting disk 18 forms one turn of a helix having a shallow pitch. Upper side 62 of cut 22 is higher than lower side 60. The importance of this fact will become apparent as the construction of the device is explained further.
Prior art Bitter magnets are made in several different ways. The specifics of the prior art construction techniques are not critical to the present invention, since the present invention could be constructed using any of the prior art techniques. However, in order to aid the understanding of those not skilled in the art, one of the prior art construction techniques will be discussed in detail:
Returning now to FIG. 1, the reader will observe that six conducting disks 18 are initially placed over tie rods 16 (the lowest part of the stack in the view). As they are stacked, each successive disk is indexed {fraction (1/15)} turn in the clockwise direction (corresponding to the fact that there are 15 tie rods 16). Turning to FIG. 3, the effect of the rotational indexing may be more readily observed. Six conducting disks 18 have been assembled to create conductor stack 30. Conducting disks 18 have also been “nested” together. The {fraction (1/15)} turn is an arbitrary figure—corresponding to the use of 15 tie rods. If 16 tie rods were used, the appropriate index could be {fraction (1/16)} turn. Rotational indexing as large as ⅓ turn is in common use, especially for smaller diameter stacks.
The disks are nested in the manner shown, so that upper side 62 of one conductor disk 18 lies over upper side 62 of the conductor disk 18 just below it. The disks in FIG. 3 are shown with a significant gap between them. The Bitter-disk assembly method squeezes the disks tightly together when the device is complete. When squeezed together, conducting disks 18 form one integral conductor having a helical shape—albeit with a very shallow pitch. Conductor stack 30 then forms a portion of one turn of the Bitter-disk magnet.
Returning now to FIG. 1, the description of the prior art device will be continued. The reader will observe that four conductor stacks 30 are shown in the assembly (in the uncompressed state). In reality, many such conductor stacks 30 will be stacked onto tie rods 16.
The desired result is to accommodate a large electrical current flowing through a helix having a shallow pitch. The desired path of current flow commences with input conductor 64 on end plate 12 (which makes contact with the underside of the lowermost conducting disk 18). A second end plate 12 (not shown) will form the upper boundary of the assembly (“sandwiching” the other components in between). The current will then exit the device through a corresponding output conductor on the upper end plate 12. Those skilled in the art will realize that if one simply stacks a number of conductor stacks 30 on the device, the electrical current will not flow in the desired helix. Rather, it will simply flow directly from the lower end plate 12 to the upper end plate 12 in a linear fashion. An additional element is required to prevent this.
Insulating disks 20 are placed within each conductor stack 30 to prevent the aforementioned linear current flow. Each insulating disk 20 is made of a material having a very high electrical resistance. The dimensional features of each insulating disk 20 (tie rod holes, cooling holes, etc.) are similar to the dimensional features of conducting disks 18. Each conductor stack 30 incorporates one insulating disk 20 nested into the stack. FIG. 1B shows a detail of this arrangement. The reader will observe the upper portion and lower portion of each insulating disk 20 (both are labeled as “20” in the view so that the reader may easily distinguish them from conducting disks 18). The reader will also observe how each insulating disk 20 nests into the helix formed by the six conducting disks 18.
FIG. 3 also illustrates this arrangement. Insulating disk 20 is placed immediately over the first conducting disk 18. It then follows the same helical pattern as the conducting disk 18. Returning now to FIG. 1, the cumulative effect of this construction will be explained. The four conductor stacks 30 shown in FIG. 1 are identical. When they are compressed together, the four insulating disks 20 will form one continuous helix through the stacked conducting disks 18. Thus, the construction disclosed forces a helical flow of electrical current through the device.
Those skilled in the art will realize that when a substantial electrical current is passed through Bitter magnet 10, strong mechanical forces are created (Lorentz forces). Significant heat is also introduced through resistive losses. Thus, the device must be able to withstand large internal mechanical forces, and it must also be able to dissipate heat. Once the entire device is assembled with the two end plates 12 in place, the end plates are mechanically forced toward each other. The lower ends of tie rods 16 are anchored in the lower end plate 12. The upper ends pass through holes in the upper end plate 12. The exposed upper ends are threaded so that a set of nuts can be threaded onto the exposed ends of tie rods 16 and tightened to draw the entire assembly tightly together. In this fashion, the device is capable of resisting the Lorentz forces, which generally tend to move the disks and other components relative to each other.
Because Bitter magnet 10 generates substantial heat during operation, natural convective cooling is generally inadequate. Forced convective cooling, using deionized water, oil, or liquid nitrogen is therefore employed. A sealed cooling jacket is created by providing an inner cylindrical wall bounded on its lower end by central hole 14 in the lower end plate 12, and bounded on its lower end by central hole 14 in the upper end plate 12. An outer cylindrical wall is provided outside the outer perimeter of the disks, extending from the lower end plate 12 to the upper end plate 12. All the components illustrated are thereby encased in a sealed chamber. The liquid is then forced into the cooling jacket, where it flows from one end of the device to the other through the aligned cooling holes 26 in the stacked disks (the cooling holes align in the conducting and insulating disks). In FIG. 1, the cooling flow would be linear from top to bottom or bottom to top.
Those skilled in the art will realize that the completed Bitter magnet 10 will generate an intense magnetic field within the cylindrical cavity within the inner cylindrical wall. Those skilled in the art will also realize that it is possible to generate an even greater magnetic field by nesting concentric Bitter-type coils. All these components are well known within the prior art.
The principle limitation of the prior art Bitter-type magnets is that they can only produce a longitudinal magnetic field—aligned with the central axis of the coil. The present invention seeks to overcome this limitation through the use of a modified Bitter magnet.